1. Field of the Invention
The present invention relates to a single crystal producing apparatus for producing a single crystal by pulling using the Czochralski (CZ) method and a single crystal producing method using the producing apparatus.
2. Description of the Related Art
As an element material of a silicon wafer used for manufacture of a semiconductor device, a silicon single crystal produced by the CZ method is frequently used. In producing the silicon single crystal by the CZ method, as is well known, a seed crystal held at a lower end of a pulling wire is immersed in a molten silicon liquid formed in a quartz crucible. The pulling wire is then raised while the shaft is rotated, thereby producing a silicon single crystal below the seed crystal.
Here, the seed crystal is a thin rod body comprising the silicon crystal of some tens of millimeters in diameter. An upper part thereof is linked with a seed holder and a lower part thereof is immersed in the molten silicon liquid. When such a seed crystal is immersed in a hot molten silicon liquid, dislocation is introduced due to thermal shock. Therefore, the seed crystal is narrowed in diameter after the crystal has been immersed in the molten silicon liquid. For a while, this state is maintained, and so-called seed contraction is implemented to ensure no dislocation of the crystal. The diameter of the seed at its narrow part is required to 5 mm or less to ensure no dislocation, and preferably is desired to be 3 mm or less.
On the other hand, a silicon single crystal, 8 inches in diameter and about 100 kg in weight produced by the CZ method, has been preferred. However, recently, the diameter of the single crystal has been further increased, and production of a silicon single crystal of 12 inches in diameter is starting. When the diameter of the single crystal is increased, of course, its weight is increased. In the case of the crystal 12 inches in diameter, the weight reaches 200 kg. This weight particularly concentrates in a seed contraction portion of the seed crystal. However, since destruction strength of silicon is about 20 kg /mm.sup.2, a seed contraction portion exceeding at least 5 mm is required to hold reliably a silicon single crystal of 200 kg in weight. Therefore, it is impossible to pull a single crystal of 12 inches in diameter from a hot molten silicon liquid pool.
As a very efficient technique eliminating this problem, making it possible to produce a 12-inch single crystal, a pulling technique independent of seed crystals is disclosed in Japanese Patent Publication No. 5-65477. In this technique, a narrowing portion is at an upper end part and this narrowing portion is additionally gripped by a chuck mechanism to pull the single crystal. The chuck mechanism is provided with a plurality of claw members for clamping the narrowing portion of the single crystal at its periphery. The plurality of claw members are raised and rotated in synchronism with the seed crystal, i.e. a seed holder and the pulling wire, thereby performing pulling of the single crystal.
In this pulling technique, the chuck mechanism carries the entire load of a single crystal, thus making it possible to produce a single crystal exceeding 12 inches. In an earlier technique disclosed in Japanese Patent Publication No. 5-65477, a chuck mechanism is integrated with a seed holder. With this structure, during chuck mechanism pulling, the chuck mechanism is completely synchronized with the seed holder to perform steps of rotating and raising. However, when a seed crystal is immersed in a hot molten liquid or before and after the immersion, the chuck mechanism approaches the hot molten liquid and is subjected to a high-temperature atmosphere exceeding 1000.degree. C. Thus, there is a problem that the hot molten liquid is contaminated by metal from the chuck mechanism. In addition, there is another problem that, even if a molybdenum material or plating is employed for the chuck mechanism to prevent such contamination, a slide portion is scorched in a high-temperature atmosphere exceeding 1000.degree. C., making it impossible to perform a gripping operation.
To solve these problems, there is provided a technique for elevating a chuck mechanism independently of a seed holder. In this technique disclosed in Japanese Laid-Open Patent Application No. 9-227282, the chuck mechanism is elevated and driven by a motor independent of the motor for elevating the pulling wire. According to this technique, there is no need for lowering the chuck mechanism to the vicinity of the hot molten liquid because an upper end part of the silicon single crystal can be grasped in the middle and the chuck mechanism can be set at a standby position in a low-temperature area. In this manner, the chuck mechanism is temporarily held in a low-temperature area distant from the hot molten liquid, and overheating of the chuck mechanism and contamination of the hot molten liquid are prevented. When the upper end part of the single crystal is gripped and the chuck mechanism is later raised in synchronism with the pulling wire, there is no problem in pulling the chuck mechanism.
However, even in the prior art disclosed in Japanese Laid-Open Patent Application No. 9-227282, it was found that overheating of the chuck mechanism and contamination of the hot molten liquid are not eliminated. These problems still occur because, when the chuck mechanism is set at a standby position higher than the grip position, the chuck mechanism must be set at the standby position near to the grip position due to an occurrence of resonation during lowering to the grip position and frequent occurrences of dislocations happen. Reasons why resonation occur during the lowering of the chuck mechanism are as follows.
In the prior art disclosed in Japanese Laid-Open Patent Application No. 9-227282, a mechanism for elevating the chuck mechanism is integrated with a mechanism for rotating the pulling wire. By this integration, the chuck mechanism is always rotated in synchronism with the pulling wire. This synchronous rotating movement of the pulling wire and the chuck mechanism is indispensable when gripping the single crystal and during subsequent pulling of the single crystal. On the other hand, when the chuck mechanism is lowered to the grip position, such movement causes the chuck mechanism to be vibrated in a horizontal direction.
In other words, the chuck mechanism is suspended by a plurality of wires for the purpose of raising it and is lowered to the grip position by feeding out these wires downwardly. The mechanism is integrated with a rotational mechanism for the pulling wire. Thus, the chuck mechanism is rotated at a constant speed identical to a rotation speed of the pulling wire during the lowering step. At this time, assuming that an effective length of a suspension wire is L, and gravity is `g`, the following Equation 1 is met.
Resonance rotation frequency=(1/2.pi.).multidot.(g/L).sup.1/2 60 rpm [Equation 1]
During lowering of the chuck mechanism, the effective length `L` of the suspension wire increases successively. Equation 1 represents that, if the rotation frequency in lowering the chuck mechanism is set at a constant speed governed by the rotation frequency of the pulling wire, resonance occurs when the effective length `L` of the suspension wire becomes a specific length. In other words, resonance occurs in the middle of the chuck mechanism being lowered to the grip position and the chuck mechanism starts vibrating greatly in the horizontal direction. Once this vibration occurs, even if the chuck mechanism is rotated in synchronism with the pulling wire, it causes great external force in the horizontal direction to be applied to the single crystal during gripping of the single crystal, producing dislocation of the single crystal or crystal drop accidents.
Thus, in the prior art disclosed in Japanese Laid-Open Patent Application No. 9-227282, in the case where the chuck mechanism is set at a standby position which is higher than the grip position, resonation occurs in the middle of its lowering from the standby position to the grip position, and dislocation frequently occurs, thus making it necessary to set the chuck mechanism standby position at the grip position. As a result, contamination of the hot molten liquid or thermal deterioration of the chuck mechanism cannot be sufficiently prevented.
The standby position is made higher with the raising of the grip position. However, if the grip position is raised, the seed contraction portion breaks upon gripping, and a single crystal may drop. The grip position is restricted to about 50% or less of the pulling length. At such a position, the chuck mechanism is distant from the hot molten liquid, but the distance is not too long. However, the standby time is long. Thus, the hot molten liquid is inevitably contaminated, so that the chuck mechanism thermally deteriorates.